There are many SSDI search engines on the web, but because each company builds its own SSDI database from the data they purchase from the Social Security Administration, the SSDI Search Engines are not identical.
I use the SSDI to find birth dates or death dates or the last residence of the person I am searching for. It gives you one more bit of information to steer you in a direction to continue your search. In order to use the search engines you have to know at least one or two bits of information about the person you are looking up. The person’s name is obvious but some clue as to where they were born or the year they were born or the date they died etc. If you have a match most SSDI’s will give up; Last Name, First Name, Social Security Number, State Issued, Birth date, Death Date, Last residence.
The issue with the SSDI is the giving of the social security number. This feature has been used often for identify theft. Now the social security number and other information you get from a SSDI will be for a dead person but for documents it is a good number. It is good for illegal aliens. It is a good source for the social security numbers of children who died young and that social security number can be used on someone’s income tax form to claim them as a dependent. One good article on the use fraudulent social security numbers and the SSDI is found here
Some SSDI providers do not give the social security number anymore, some still do. My father died in 2006 so I entered some information on him in the ancestry.com search engine and no social security number was shown when his data was found. I entered some information into the New England Historic Society search engine and it gave the same information from ancestry.com plus the social security number.
So the Social Security Death Index is a handy Genealogical tool. Like any data base it can be used by someone for abuse